@media screen and (max-width: 767px) {
  body {
    font-size: 90%;
    line-height: 1.6;
  }
  #contents, header, footer, nav {
    width: 100%;
    padding: 0 10px;
    box-sizing: border-box;
  }
  nav ul {
    display: block;
    text-align: center;
  }
  nav ul li {
    width: 500px;  /* ← 横幅を広げる */
  }
  nav ul li a {
    display: block;
    width: 215px;
    height: 60px;
    line-height: 60px;
    text-align: center;
    background-size: cover;
    background-position: left center;
    color: #fff;
    font-weight: bold;
  }
  #main {
    float: none;
    width: 100%;
  }

  #sub {
    float: none;
    width: 100%;
    margin-top: 20px;
  }

  img {
    max-width: 100%;
    height: auto;
  }
}